1. Field of the Invention
The present invention relates to a video camera system comprising a camera and a video cassette recorder which are integral with each other, and more particularly to an automatic focusing apparatus of such a video camera system and an automatic focusing method.
2. Description of the Prior Art
As well known, such a video camera system which is called a camcorder comprises mainly a camera unit and a video cassette recorder unit. Generally, the camera unit includes a focusing lens for adjusting a focus of an object. The focusing lens is adapted to move in response to a focusing signal generated according to a level of a luminance signal. By the movement of the focusing lens, the focus of the object is adjusted.
Referring to FIGS. 1a and 1b, there is illustrated a conventional video camera system. In the drawings, only a camera unit of the video camera system equipped with a focusing apparatus is shown.
As shown in FIG. 1a, the focusing apparatus comprises a focusing lens 1 for adjusting a focus of an object to be shot, an iris 2 for controlling the amount of incident light according to the illuminance of surroundings, and a charge coupled device (CCD) 4 for converting an optical video signal indicative of an image of the object and received via the focusing lens 1 and the iris 2, into an electrical video signal under a control of a CCD driving circuit 3. The focusing apparatus also comprises a video signal processing circuit 5 for processing an output signal from the CCD 4 and generating color difference signals R-Y (red luminance signal) and B-Y (blue luminance signal) and a luminance signal Y, an encoder 6 for encoding output signals from the video signal processing circuit 5 and outputting a composite video signal, a buffer 7 for buffering the luminance signal Y from the video signal processing circuit 5, a band-pass filter 8 for passing a particular frequency band of the luminance signal Y outputted from the buffer 7, and an analog/digital converter 9 for converting an output signal from the filter 8 into a digital signal. Also, a focusing signal generating circuit 10 is provided which is adapted to integrate an output signal from the analog/digital converter 9 by the unit of one horizontal interval, sum integrated values obtained by the integration and output the result as a focusing signal. The focusing apparatus also comprises a control circuit 11 (generally, a microcomputer) for generating a control signal for focusing, according to an output signal from the focusing signal generating circuit 10 and a motor driving circuit 12 for driving a focusing motor 13 according to the control signal from the control circuit 11 and thus moving the focusing lens 1 for focusing.
Now, operation of the focusing apparatus with the above-mentioned construction of FIG. 1a will be described.
As an object is shot by the video camera system which is powered on, an optical video signal indicative of an image of the object is fed to the CCD 4 via the focusing lens 1 and the iris 2. Upon receiving the optical video signal, the CCD 4 is enabled under a control of the CCD driving circuit 3 so that it converts the received optical video signal into an electrical video signal which is, in turn, sent to the video signal processing circuit 5. The video signal processing circuit 5 processes the received video signal to device it into color difference signals R-Y and B-Y and a luminance signal Y. The encoder 6 receives the color difference signals R-Y and B-Y and luminance signal Y from the video signal processing circuit 5 and encodes them to generate a composite video signal.
The luminance signal Y from the video signal processing circuit 5 is also sent to the buffer 7 in which it is buffered and then sent to the band-pass filter 8. The band-pass filter 8 outputs only a particular frequency band of the luminance signal Y which is, in turn, sent to the analog/digital converter 9 and converted into a digital signal. The analog/digital converter 9 sends the digital signal to the focusing signal generating circuit 10.
The focusing signal generating circuit 10 integrates the output signal from the analog/digital converter 9 by the unit of one horizontal interval and sums the resultant integrated values. The resultant summed value is then sent as a focusing signal to the control circuit 11.
By the focusing signal from the focusing signal generating circuit 10, the control circuit 11 generates a control signal which is, in turn, sent to the focusing motor driving circuit 12. The motor driving circuit 12 energizes the focusing motor 13, according to the control signal. By the driving of the motor 13, the focusing lens 1 moves to adjust the focus of the object.
FIG. 1b is a detailed block diagram of the COD driving circuit shown in FIG. 1a. As shown in FIG. 1b, the COD driving circuit comprises an oscillator 3a for generating a signal with a frequency of 28.6363 MHz, a timing signal generator 3b for frequency-dividing the signal of 28.6363 MHz by 2, a synchronizing signal generator 3c for generating a vertical synchronizing signal and a horizontal synchronizing signal and feeding back them to the timing signal generator 3b, in response to an output signal from the timing signal generator 3b, and a vertical-side driver 3d for transferring the vertical synchronizing signal from the timing signal generator 3b to the CCD 4. The timing signal generator 3b also sends directly the horizontal synchronizing signal from the synchronizing signal generator 3c to the CCD 4.
However, the conventional video camera system with the construction of FIG. 1a has the following problems.
First, where colors of white and black are displayed on a screen, as shown in FIG. 2a, the buffer 7 of FIG. 1a generates a luminance signal shown in FIG. 2b, correspondingly to the colors of FIG. 2a. When colors of white and gray are displayed, as shown in FIG. 3a, it generates a luminance signal shown in FIG. 3b. These luminance signals are then filtered in the band-pass filter 8, thereby producing luminance signal s shown in FIGS. 2c and 3c, respectively. Referring to FIGS. 2c and 3c, it can be found that the boundary between white and black exhibits a higher peak value than the boundary between white and gray. However, the conventional video camera system of FIG. 1a converts such luminance signal s into digital signals through its analog/digital converter 9, without taking the peak values into consideration. These digital signal s are integrated in the focusing signal generating circuit 10 which, in turn, sums the results obtained by the integration and generates a focusing signal based on the summed result. As a result, the boundary between white and black is actually more distinct than the boundary between white and gray, due to a difference in level therebetween, as apparent from FIGS. 2band 3b. However, the level of the focusing signal for the boundary between white and gray may often be higher than that of the focusing signal for the boundary between white and black, since the levels of the focusing signals outputted from the focusing signal generator 10 of FIG. 1b correspond to shaded areas of FIGS. 2c and 3c, respectively. The shaded area of FIG. 3c is larger than that of FIG. 2c. As a result, the focusing is carried out in a manner that the boundary between white and gray becomes more distinct than the boundary between white and black, even though it should be carried out in a manner that the boundary between white and black becomes more distinct than the boundary between white and gray. This results in degradations in quality of a video signal indicative of an object image and thereby in screen picture quality. Consequently, it causes the user to be unpleased and the reliance for the system to be degraded.
Second, the luminance signal from the video signal processing circuit 5 of FIG. 1a passing through the buffer 7 and the band-pass filter 8 is outputted at a level variable depending on a zooming condition of the video camera system. For example, the level of luminance signal varies gently at a tele-zooming mode of the system, according to the driving of the focusing motor 13, as shown in FIG. 4. On the other hand, the level of luminance signal varies sharply at a wide-zooming mode of the system. However, the conventional video camera system performs the focusing by driving the focusing motor 13 at a constant speed, without taking the zooming condition into consideration. At the wide-zooming mode, a precise focused position can be easily found, in that the variation in luminance signal level is gentle. At the tele-zooming mode, however, an oscillation phenomenon may occur upon focusing. That is, the focusing lens passes by the precise focused position and then moves back to the precise focused position, since the driving speed of the focusing motor 13 is high due to the sharp variation in luminance level at the tele-zooming mode. Such an oscillation phenomenon causes the quality of video signals to be degraded and thereby the user to be unpleased.
Third, where a frame corresponding to a video signal indicative of an object image is laterally dividedinto white and black in the video camera system of FIG. 1a, as shown in FIG. 5a, the boundary between white and black is detected within one horizontal interval H1, as shown in FIG. 5b. Also, a high frequency component at the boundary is detected. By the detected high frequency component, it is possible to achieve a precise focusing, as shown in FIG. 5c. Where the frame is vertically divided into white and black, as shown in FIG. 6a, however, the boundary between:white and black may be liable to be present beyond a focusing scope, as shown in FIG. 6b. In this case, no high frequency component at the boundary is detected, thereby disabling the precise focusing.
Fourth, the conventional video camera system of FIG. 1a detects only the level of the luminance signal and achieves the focusing, based on the detected luminance signal level. As a result, where an object bearing the same kind of colors with different concentrations or different kinds of colors with the same luminance signal level is shot, the focusing can not be precisely achieved.